1. Field of the Invention
The present invention generally relates to a method and system for generating a feedback signal in optical disc drives, and more particularly to a method and system to use the pseudo-jitter for generating the feedback signal.
2. Description of the Prior Art
In conventional optical disc drives, the jitter is normally an index to determine the performance of the optical disc drives. The optical system of the pickup head is an important factor for determining the performance of the optical disc drives. In other words, if the pickup head has an improper design, the jitter will be higher than as usual. By this, the optical disc drives normally feedback a jitter signal to the pickup head for correcting the optical system. For instance, in blue-ray disc drives, in order to compensate the spherical aberration, which caused by the different thickness or the double layer of disc, the jitter signal is normally used to generate a feedback signal for compensating the error that caused by the spherical aberration.
FIG. 1 shows a conventional feedback system of optical drives. The pickup head 101 is for reading radio frequency (RF) signal. The filter 102 is for pre-processing the RF signal. The analog-to-digital converter (ADC) 103 samples the RF signal and converts it to a digital signal. After the equalizer 104 equalizes and compensates the digital signal, the output of the equalizer 104 transmits into two paths. One path is to phase-lock loop (PLL) 109 and the PLL 109 feedbacks signal to ADC 103 and equalizer 104 for providing the clock, for processing digital signal. The other one path of the equalizer 104 is to the digital-to-analog converter (DAC) 105, which is for transferring digital signal into analog signal. Then the analog signal is processed by the low-pass filter (LPF) 106 and the slicer 107 and inputted to jitter meter 108. The jitter meter 108 detects the jitter by comparison of the outputting signal of the slicer 107 and generated clock of the PLL 109. The jitter detecting is complexity because the jitter detecting is based on analog signal. The detected jitter is feedback to the pickup head for controlling the focus area of the pickup head.
The jitter detecting is as shown in FIG. 2. The axes 303 and 304 represent a reference level (hereinafter, for example a zero level). The waveform 301 represents a waveform with zero jitter observing from the point A, which is shown in FIG. 1. The waveform 302 represents a waveform with jitter, which is not zero observing from the point A. By comparison of the waveform 301 and 302, there exists a phase shift 305 between the point 3011 and 3021. The phase shift 305 here is so-called jitter and it is normally defined in standard. In other words, the jitter detecting must be based on analog processing. The prior jitter meter detects the phase shift, transfers the phase shift to voltages and finally feedbacks to the pickup head for controlling the focus area.
According to the previous mentioned, the feedback signal could not be digitalized in prior art. There are some disadvantages such as: rebuilding original signal difficulty, signal delay and signal distortion due to converting digital-to-analog and analog-to-digital many times. For example: in order to compensate the spherical aberration, which caused by the different thickness or the double layer of disc, the jitter signal is normally used to generate a feedback signal for compensating the error that caused by the spherical aberration. However, the feedback circuit described above may lead to a large amount of cost and problems of integrating in blue-ray disc drive.